Blackleg is a serious disease of canola that can result in significant yield loss in susceptible varieties in Canada, Europe, and Australia. Blackleg is the most common name, but stem canker or Phoma stem canker are also used for this disease. It is caused by the fungus Leptosphaeria maculans. In Saskatchewan a highly aggressive strain of the blackleg fungus was first detected in 1975, but now occurred in most canola-growing areas in western Canada.
Canola disease field surveys are conducted annually in the main canola production regions of western Canada. Disease surveys from recent years indicate that blackleg is commonly found in 35 to 55 percent of crops surveyed. Average disease incidence values (percentage of plants showing blackleg symptoms) are typically one percent for basal stem cankers and three percent for lesions occurring elsewhere on the stem. Lesions occurring elsewhere on the stem will have less impact on seed yield and quality than will basal stem cankers. The highest incidence values are often observed in crops that had received hail damage.
As is indicated in the disease surveys, it is not unusual to observe blackleg symptoms in canola crops, even when resistant varieties are being grown. However, to prevent blackleg from negatively impacting seed yield and quality, it is important to be familiar with blackleg symptoms, the disease cycle, and disease management practices.
Blackleg is the most serious disease of canola in Australia. The severity of blackleg has risen in recent years due to increased acreage and intensity of production. Although not common, yield losses of 50 percent and greater have been recorded in some seasons with up to 90 percent yield loss occurring in cases where L. maculans has overcome major blackleg resistance genes within certain varieties.
In the autumn and winter, rainfall triggers spore release from the stubble. Within two weeks of spores landing on canola cotyledons and young leaves, clearly visible off-white coloured lesions develop, within the lesion pycnidial fruiting bodies (dark coloured dots) release rain-splashed spores. Blackleg infections may occur on cotyledons, leaves, stems and pods. The plant is susceptible to blackleg infection from the seedling to pod-set stages. Lesions occurring on the leaves are dirty white and are round to irregularly. On stems, blackleg lesions can be quite variable, but are usually found at the base of the stem, or at points of leaf attachment. Once a lesion has formed, the fungus grows within the plants vascular system to the crown where it causes the crown of the plant to rot, resulting in a canker. Severe canker will sever the roots from the stem, whereas a less severe infection will result in internal infection of the crown restricting water and nutrient flow within the plant. Stem lesions may be up to several inches in length, and are usually white or grey with a dark border. Stem lesions may also appear as a general blackening at the base. Severe infection usually results in a dry rot or canker at the base of the stem. The stem becomes girdled and, as plants ripen prematurely, the crop is more likely to lodge. Seed may be shriveled and pods shatter easily at harvest, resulting in seed loss.
In recent years blackleg symptoms have also been found in the plant roots, this root infection in severe cases appears to cause the entire plant to die prematurely. The root rot form of the disease is caused by the same blackleg strains that cause the stem canker and management practices to control normal blackleg are the same for the root rot form of the disease.
With increasing acres of canola and often tighter rotations, blackleg disease has again started to become an important yield and quality reducing disease.
Varietal resistance is the best defense, to date, against blackleg disease. The development of blackleg resistant varieties has lead to reduced economic losses due to blackleg. As a result, many farmers are growing resistant varieties and they have been able to get away with tighter and tighter rotations. Preliminary observations from the 2010 canola disease survey show that in Manitoba the R-rated varieties are starting to show higher incidence and severity of the blackleg disease. L. maculans has developed new variants and these new variants are now able to infect the earlier R-rated canola varieties.
When the blackleg resistant varieties were first developed, researchers categorized the pathogen into “PG” patho-groups, which were based on the infectivity of an isolate on 3 varieties—Westar, Glacier and Quinta. With changing blackleg populations, the PG classification is insufficient to describe newer variants. This means the PG system cannot identify variability among isolates as a result of sources of resistance not found in ‘Quinta’ and ‘Glacier’. Based on molecular markers, pathogen isolates have been able to be classified into PG groups, much quicker than the traditional plant inoculation method (Dusabenyagasani, M., and Fernando, W. G. D. 2008).
In a newer approach, by using varieties or lines of Brassica spp. carrying 14 specific resistance genes, new isolates of L. maculans can be differentiated or characterized into races based on the reactions observed. Rimmer (2007) had reported these 14 genes of specific resistance, designated Rlm 1 to Rlm 10 and LepR1 to LepR4. A study of 96 western Canadian isolates of blackleg fungus, using ten resistance genes indicated considerable variation in the pathogen population for many of these genes (Kutcher et al. 2010).
There are numerous steps in the development of any novel, desirable plant germplasm. Plant breeding programs combine desirable traits from two or more cultivars or various broad-based sources into breeding pools, from which cultivars are developed by selfing and selection of desired phenotypes. The new cultivars are evaluated to determine which have commercial potential. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possess the traits to meet the program goals. The goal is to combine in a single variety an improved combination of desirable traits from the parental germplasm. These important traits may include higher seed yield, resistance to diseases and insects, better stems and roots, tolerance to drought and heat, and better agronomic quality.
Choice of breeding or selection methods depends on the mode of plant reproduction, the heritability of the trait(s) being improved, and the type of cultivar used commercially (e.g., F1 hybrid cultivar, pureline cultivar, etc.). For highly heritable traits, a choice of superior individual plants evaluated at a single location may be effective, whereas for traits with low heritability, selection should be based on mean values obtained from replicated evaluations of families of related plants. Popular selection methods commonly include pedigree selection, modified pedigree selection, mass selection, and recurrent selection.
The complexity of inheritance influences the choice of the breeding method. Backcross breeding is used to transfer favorable alleles of one or a few genes for a highly heritable trait into a desirable cultivar. This approach has been used extensively for breeding disease-resistant cultivars. Various recurrent selection techniques are used to improve quantitatively-inherited traits controlled by numerous genes. The use of recurrent selection in self-pollinating crops depends on the ease of pollination, the frequency of successful hybrids from each pollination, and the number of hybrid offspring from each successful cross.
Each breeding program should include a periodic, objective evaluation of the efficiency of the breeding procedure. Evaluation criteria vary depending on the goal and objectives, but should include gain from selection per year (based on comparisons to an appropriate standard), overall value of the advanced breeding lines, and the number of successful cultivars produced per unit of input (e.g., per year, per dollar expended, etc.). Promising advanced breeding lines are then thoroughly tested and compared to appropriate standards in environments representative of the commercial target area(s) for three or more years. Candidates for new commercial cultivars are selected from among the best lines; those still deficient in a few traits may be used as parents to produce new populations for further selection. These processes, which lead to the final step of marketing and distribution, usually take from 8 to 12 years from the time the first cross is made. Therefore, development of new cultivars is a time-consuming process that requires precise forward planning, efficient use of resources, and a minimum of changes in direction.
A most difficult task in plant breeding is the identification of individuals that are genetically superior. One method of identifying a superior plant is to observe its performance relative to other experimental plants and to a widely grown standard cultivar. If a single observation is inconclusive, replicated observations provide a better estimate of its genetic worth. This task is so difficult, because (for most traits) the true genotypic value is masked by other confounding plant traits or environmental factors.
The goal of canola plant breeding is to develop new, unique, and superior canola cultivars and hybrids. The breeder initially selects and crosses two or more parental lines, followed by repeated selfing and selection, producing many new genetic combinations. The breeder can theoretically generate billions of different genetic combinations via crossing, selfing, and mutagenesis. Such a breeder has no direct control of the process at the cellular level. Therefore, two breeders will never develop the same line, or even very similar lines, having the same canola traits.
Each year, the plant breeder selects the germplasm to advance to the next generation. This germplasm is grown under unique and different geographical, climatic, and soil conditions. Further selections are then made, during and at the end of the growing season. The cultivars that are developed are unpredictable. This unpredictability is due to the breeder's selection, which occurs in unique environments, and which allows no control at the DNA level (using conventional breeding procedures), with millions of different possible genetic combinations being generated. A breeder of ordinary skill in the art cannot predict the final resulting lines he develops, except possibly in a very gross and general fashion. Similarly, the same breeder cannot produce the same cultivar twice by using the exact same original parents and the same selection techniques. This unpredictability results in the expenditure of large amounts of resources, monetary and otherwise, to develop superior new canola cultivars.
The development of new canola cultivars requires the development and selection of canola varieties, crossing of these varieties, and selection of superior hybrid crosses. Hybrid seed is produced by manual crosses between selected male-fertile parents, or by using male sterility systems. These hybrids are selected for certain single gene traits (e.g., pod color, flower color, pubescence color, and herbicide resistance) that indicate that the seed is truly a hybrid. Data on parental lines, as well as the phenotype of the hybrid, influence the breeder's decision regarding whether to continue with the specific hybrid cross.
Pedigree breeding is used commonly for the improvement of self-pollinating crops. In pedigree breeding, two parents that possess favorable, complementary traits are crossed to produce F1 progeny. An F2 population is produced by selfing one or several plants from the F1 progeny generation. Selection of the best individuals may begin in the F2 population; then, beginning in the F3, the best individuals in the best families are selected. To improve the effectiveness of selection for traits with low heritability, replicated testing of families can begin in the F4 generation. At an advanced stage of inbreeding (e.g., F6 or F7), the best lines or mixtures of lines with similar phenotypes are tested for potential release as new cultivars. Mass and recurrent selections can be used to improve populations of either self- or cross-pollinating crops. A genetically variable population of heterozygous individuals may be either identified or created by intercrossing several different parents. The best plants may be selected based on individual superiority, outstanding progeny, or excellent combining ability. The selected plants are intercrossed to produce a new population, in which further cycles of selection may be continued.
Backcross breeding has been used to transfer genes for a simply- and highly-heritable trait into a desirable homozygous cultivar, or inbred line, which is the recurrent parent. The source of the trait to be transferred is the “donor parent.” The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar), and the desirable trait transferred from the donor parent. After the initial cross, individuals possessing the phenotype of the donor parent are selected, and repeatedly crossed (backcrossed) to the recurrent parent. The resulting plant is expected to have the attributes of the recurrent parent and the desirable trait transferred from the donor parent.
In canola breeding, the “single-seed descent procedure” refers to the planting of a segregating population, followed by harvesting a sample of one seed per resulting plant, and using the harvested one-seed sample to plant the next generation. When the population has been advanced from the F2 generation to the desired level of inbreeding, the plants from which lines are derived will each trace to different F2 individuals. The number of plants in a population declines each generation, due to failure of some seeds to germinate or some plants to produce at least one seed. As a result, not all of the F2 plants originally sampled in the population will be represented by a progeny when generation advance is completed.
In a multiple-seed procedure, canola breeders commonly harvest seeds from each plant in a population and thresh them together to form a bulk. Part of the bulk is used to plant the next generation, and part is put in reserve. This procedure has been referred to as modified single-seed descent. The multiple-seed procedure has been used to save labor involved in the harvest. It is considerably faster to remove seeds with a machine, than to remove one seed from each by hand for the single-seed procedure. The multiple-seed procedure also makes it possible to plant the same number of seeds of a population for each generation of inbreeding. Enough seeds are harvested to compensate for the number of plants that did not germinate or produce seed.
Proper testing should detect any major faults and establish the level of superiority or improvement of a new cultivar over current cultivars. In addition to showing superior performance, there should be a demand for a new cultivar that is compatible with industry standards, or that creates a new market. The testing preceding release of a new cultivar should take into consideration research and development costs as well as technical superiority of the final cultivar. The introduction of a new cultivar can incur additional costs to the seed producer, the grower, the processor, and the consumer due to special required advertising and marketing, altered seed and commercial production practices, and new product utilization. For seed-propagated cultivars, it must be feasible to produce seed easily and economically.
It is the goal of the plant breeder to select plants and enrich the plant population for individuals that have desired traits, for example, disease resistance or blackleg resistance, leading ultimately to increased agricultural productivity. Consistent with the foregoing, a continuing goal of canola breeders is to develop stable, high-yielding cultivars that are agronomically sound. Current goals include maximization of the amount of grain produced on the land used, and the supply of food for both animals and humans. To accomplish these goals, the canola breeder must select and develop canola plants that have traits that result in superior cultivars, and do so in the most cost-effective manner. Molecular markers may be used in the process of marker-assisted selection (MAS) to aid in the identification and selection of individuals or families of individuals that possess inherited attributes that are linked to the markers.